1. Field of Invention:
This invention relates to communications protocols. Specifically, the present invention relates to protocols employed for high-speed, long-latency, asymmetric, and bit-error prone data links.
2. Description of the Related Art:
Communications protocols are employed in various demanding applications including aircraft-to-ground communications and communications over satellite relays. Such applications require accurate and reliable protocols that maximize the use of available bandwidth over data links with long latencies (which are round trip signal delay times from sender to receiver), bit errors, and asymmetries (which include different downlink and uplink transmission rates).
In aircraft-to-ground and satellite-to-ground communications, control information is often transferred over the uplink from the ground to the aircraft or satellite while much data is transferred over the downlink from the aircraft to the ground. In such communications systems, downlink traffic from the aircraft or satellite to ground may be 1000 times greater than uplink traffic from ground to the satellite or aircraft. Data rates of hundreds of megabits per second are common for the downlink, while data rates of tens of kilobits per second are common for the up link. Such asymmetries reduce channel throughput and bandwidth utilization of communications systems employing conventional protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP).
TCP/IP performance significantly degrades as bit errors, latencies, or asymmetries increase. Hence, TCP/IP is inappropriate for many applications. For example, TCP/IP requires an acknowledge packet for every received data packet.
Consequently, TCP/IP acknowledgement packets readily saturate the uplink of highly asymmetric channels.
LFNTCP aims to improve the accuracy of TCP/IP over high-speed asymmetric links. Unfortunately, LFNTCP still performs unacceptably in satellite relay tests. High data rates and large round trip delays reduce TCP or LFNTCP channel throughput to a small fraction of available bandwidth.
Communications systems employing TCP/IP or LFNTCP over high-speed asymmetric data links often use only a small fraction of expensive allotted bandwidth. The wasted bandwidth increases costs to effectively communicate data over the link, as more bandwidth is purchased to transfer more information.
Hence, a need exists in the art for an efficient and reliable protocol that maximizes the use of available bandwidth allocated for high-speed, asymmetric, long-latency, and/or error prone communications links. There exists a further need for an efficient communications system for accommodating the protocol.
The need in the art is addressed by the system for efficiently and reliably communicating over a high-speed asymmetric communications link of the present invention. In the illustrative embodiment, the inventive system is adapted for use with aircraft-to ground and satellite relay communications links. The system includes a first mechanism for establishing contact between a first device and a second device over a channel. A second mechanism delivers data packets over the channel from the first device to the second device. Each packet is associated with a window of packets. The window of packets includes a given number of bytes. Packets may vary in size. A third mechanism selectively employs the second mechanism to re-send data packets not received by the second device after each window of packets via a response message that encapsulates multiple missing packet identifications.
In a specific embodiment, the window of packets is sized in accordance with communications link bandwidth and the round trip delay time of the communications link. The first mechanism includes Transmission Control Protocol/Internet Protocol (TCP/IP) functionality on the first device and the second device for establishing a first TCP/IP link from the second device to the first device. The first mechanism also includes Universal Datagram Protocol (UDP) functionality on the first device and the second device for transferring UDP packets from the first device to the second device.
The third mechanism sends acknowledgement messages from the second device to the first device specifying the packets not received by the second device. The system further includes a fourth mechanism for selectively disabling the second mechanism when first device does not receive one of the acknowledgement messages after a predetermined time interval. The predetermined time interval is a function of a window timeout variable. The predetermined function is (M)xc3x97(window timeout), where M is approximately two. The window timeout is greater than N multiplied by ack-window worth of data divided by the data rate of the communications link between the first device and the second device, where N is an integer greater than 1. N is between 3 and 10.
Each packet includes a header having a file parameter and a sequence parameter that specify a file associated with each of the packets and a position in the file of each of the packets, respectively. The system further includes a fifth mechanism for assembling packets received by the second device via the second mechanism based on the file parameter and the sequence parameter.
The first device includes a transmit buffer for transmitting the packets, and the second device includes a receive buffer for receiving the packets. The size of the transmit buffer is a function of round trip signal travel time between the first device and the second device, a size associated with the acknowledge-window and a maximum data rate associated with the channel. The transmit buffer is approximately 50 megabytes for a 100 megabits per second channel. The size of the receive buffer is approximately equivalent to the size of the transmit buffer.
The novel design of the present invention is facilitated by the third means and the receive buffer of the second device. By employing a large receive buffer and sending negative acknowledgement messages indicating missing packets associated with prior missing packets after each window of packets, enhanced reliability and communications link throughput is achieved. This partly results from the fact that lost packets may be reinserted into the data stream without halting transmissions, and the fact that fewer acknowledgement messages (and fewer associated message headers and corresponding message overhead) are required to reliably communicate over the link. This reduces or eliminates the possibility of uplink saturation due to high data rates on the downlink.